#include<iostream>
#include<cstdio>
#include<queue>
using namespace std;
const int inf=1<<30;
int n,m,q,s,t,mf,mc,px[200005],py[200005];int fx[4][2]={{0,1},{0,-1},{1,0},{-1,0}};
int tot=1,fir[400005],cur[400005],nxt[2000005],to[2000005],cap[2000005],val[2000005];
int vis[400005],rq[400005],dis[400005];
int ch(int x,int y){return (x-1)*m+y;}
void ins(int x,int y,int z,int w)
{
	nxt[++tot]=fir[x];
	fir[x]=tot;
	to[tot]=y;
	cap[tot]=z;val[tot]=w;
}
bool spfa()
{
	for(int i=1;i<=t;i++) dis[i]=inf,cur[i]=fir[i],vis[i]=rq[i]=0;
	queue<int> q;q.push(s);vis[s]=0,rq[s]=1;dis[s]=0;
	while(!q.empty())
	{
		int now=q.front();q.pop();
		for(int i=fir[now];i;i=nxt[i])
			if(cap[i]&&dis[to[i]]>dis[now]+val[i])
			{
				dis[to[i]]=dis[now]+val[i];
				if(!vis[to[i]])
				{
					vis[to[i]]=1;
					rq[to[i]]++;
					q.push(to[i]);
					if(rq[to[i]]>t) return 0;
				}
			}
		vis[now]=0;
	}
	return (dis[t]!=inf);
}
int dfs(int now,int sum)
{
	if(vis[now]||!sum) return 0;
	if(now==t) return sum;
	vis[now]=1;int res=0,k=0;
	for(int i=cur[now];i&&sum;i=nxt[i])
	{
		cur[now]=i;
		if(!vis[to[i]]&&cap[i]&&dis[to[i]]==dis[now]+val[i])
		{
			k=dfs(to[i],min(cap[i],sum));
			if(!k) dis[to[i]]=inf;
			else
			{
				cap[i]-=k;cap[i^1]+=k;
				res+=k;sum-=k;mc+=val[i];
			}
		}
	}
	vis[now]=0;return res;
}
int main()
{
	ios::sync_with_stdio(false);
	while(cin>>n>>m>>q)
	{
		for(int i=1;i<=q;i++) cin>>px[i]>>py[i];
		s=2*n*m+1;t=2*n*m+2;mf=mc=0;
		for(int i=1;i<=n;i++)
			for(int j=1;j<=m;j++)
			{
				int cp=ch(i,j);
				ins(cp,cp+n*m,1,0),ins(cp+n*m,cp,0,0);
				for(int k=0;k<4;k++)
				{
					int tx=i+fx[k][0],ty=j+fx[k][1],np=ch(tx,ty);
					if(tx<=0||ty<=0||tx>n||ty>m) continue;
					ins(cp+n*m,np,1,1),ins(np,cp+n*m,0,-1);
				}
				if(i==1||i==n||j==1||j==m) ins(cp+n*m,t,1,0),ins(t,cp+n*m,0,0);
			}
		for(int i=1;i<=q;i++)
		{
			int cp=ch(px[i],py[i]);
			ins(s,cp,1,0),ins(cp,s,0,0);
		}
		while(spfa()) mf+=dfs(s,inf);
		if(mf<q) cout<<"-1\n";
		else cout<<mc<<"\n";
		for(int i=1;i<=t;i++) fir[i]=cur[i]=0;tot=1;
	}
	return 0;
}